Nuclear seam generators which have an economizer or preheater section include a physical barrier or flow divider plate to separate the hot inlet fluid from the colder outlet flow areas of the heat exchanger tube bundle on the secondary side of the tubesheet.
A typical nuclear steam generator is a heat exchanger vessel containing a tube sheet which separates a primary side cavity and a secondary side cavity. A bundle of U-shaped tubes is provided with each tube end mounted to extend through the tubesheet in fluid communication with the primary side cavity the to extend into the secondary side cavity to provide a heat exchanging relationship between fluid in the tubes and fluid in the secondary side cavity. The secondary side cavity flow divider plate extents across the vessel and perpendicularly from the tubesheet. Together with other flow directing baffle plates it makes up the economizer or preheater section of the heat exchanger. The economizer improves heat transfer by preheating the incoming feedwater using the low temperature heat transfer portion of the U-tubes, i.e., adjacent where they pass fluid outwardly into the primary cavity outlet side which is separated from the primary cavity inlet side by a divider plate.
The steam generator includes a handhole through the vessel wall adjacent to the tubesheet and divider plate for maintenance and cleaning n the secondary side cavity. Use of the handhole is typically to remove loose parts and debris from be annulus between the vessel inside wall and the tube bundle and to remove sludge on the tubesheet between he tubes, as by lancing.
When an economizer divider plate is present in the steam generator, it obstructs physical access for inspection and maintenance by effectively creating two distinct secondary side cavity portions which would normally require separate handholes for access. A solution to this problem has been to provide the handholes 80.degree. apart at each end of the divider plate in alignment with the plate. A portion of the plate called a "notch" has been removed from the plate end opposite both of the handholes such that the 360.degree. circumference around the tube bundle becomes accessible. During operation, the divider plate notches must be blocked to prevent cross flow from one side of the flow divider plate to the other. The purpose of the flow blocker of the improved heat exchanger of the invention is to seal the notches while the steam generator is in operation yet to be removable for maintenance or inspection.
In providing the flow blocker for use in a nuclear steam generator having an economizer, various design requirements were considered. One design requirement was that the flow blocker must be easy to install and disassemble. Another was that it has to be stable during operation of the steam generator to eliminate problems from vibration and flow loads. A further requirement was the accomplishment of its primary purpose of effectively blocking substantial bypass flow from one side of the divider plate to the other during operation. No prior are device accomplished these objectives.